In recent years, a shallow trench isolation (STI) structure including an air gap has been proposed in order to suppress intensification of a Yupin/Enda effect due to miniaturization of integrated circuit patterns. Such an STI structure is formed, for example, by burying a sacrificial layer in an isolation trench, forming a layer to serve as the upper surface of the air gap on the sacrificial layer, and then removing the sacrificial layer by etching. However, this method has a problem that it is difficult to form a large air gap in a case where the tolerance of the sacrificial layer to the etching is high.